1. Field of the Invention
The present invention relates generally to a method and apparatus for correcting inaccuracies in an optical scanner, and more particularly, to a method and apparatus for compensating for static and dynamic inaccuracies in an optical scanner.
2. Description of the Related Art
In the fabrication of modem semiconductor integrated circuits, inspection systems are used to inspect semiconductor wafers, photomasks, reticles, and other surfaces. A typical surface inspection system detects light that is scattered, reflected, or otherwise modified from a small area of an inspected surface. In order to detect errors or anomalies, data from the inspection system, in the form of a quantity of light detected, for example, may be compared with some kind of reference data. The reference data may come from a database, from another portion of the surface being inspected, or from other suitable sources.
Erroneous detection of errors or anomalies can occur when the data for a sampled surface area is not matched correctly with reference data. One source of such errors can occur when the optical system used in scanning the inspected surface is misaligned.
FIG. 1 shows a general example of an inspection system, in which an article 10 being inspected is placed on an inspection stage 20. A light source 60, which may be a laser or any suitable light source, provides its output to an acousto-optic deflector (AOD) 70, which in turn causes a light beam 55 to be output. Control of AOD 70, which for example may be accomplished, in whole or in part, by RF generator 80 under entire or partial control of a stage control 25, causes the light beam 55 to scan across the surface of the article 10. Other types of systems for causing the light beam 55 to scan as indicated may be used.
In this type of inspection system, the light beam 55 generally moves in the direction of a first axis, while the stage 20 moves the article 10 along a second, generally perpendicular axis. Relative movement between the light beam 55 and the article 10 in these respective axes may be accomplished in any known or desired manner. For example, the light beam 55 may move, and the article 10 (or stage 20) may remain stationary; the light beam 55 may remain stationary, and the article 10 (or stage 20) may move; or both the light beam and the article (stage) may move. For purposes of the following discussion, it will be assumed that the light beam scans the surface of the article 10 along the X axis, while the inspection stage moves along the Y axis.
In FIG. 1, a stage control 25 controls movement of the stage 20 in the Y axis. AOD 70 causes the light beam 55, generated by light source 60 and under control of an RF generator 80, to scan in the direction of the X axis. The RF generator 80 communicates with stage control 25 to control the scanning of light beam 55 in accordance with the movement of the stage 20.
FIG. 2 depicts generally an ideal relative movement between the stage 20 and the light beam 55. That is, ideally, in the configuration shown, for a given article 10 being inspected, the stage 20 always has the article 10 positioned precisely on it, and always begins to move at the same point along the Y axis, while the light beam 55 always begins a scan line at the same point along the X axis. With this kind of perfect alignment, the light information produced by the scan at a given position (coordinate pair) on the article will always match the data at the corresponding coordinate pair in the reference data.
Obviously, misalignment along either the X axis or the Y axis can cause misregistration, which could be sufficient to cause scan data to be matched incorrectly with reference data. As a result, an error could be misidentified in at least one of two ways. Either the system may identify a non-existent error; or the system may fail to identify an error. Accordingly, one problem that needs to be addressed is the non-ideal nature of the (x, y) grid over which the scanning occurs.
Imperfections in the (x, y) grid can result from various problems. For example, in certain kinds of inspection stage movement systems, incremental stage movements can be sufficiently imprecise to cause misalignment, with resulting inaccuracy in the position of the stage 20 relative to the light beam 55. For example, there may be an instruction to the stage 20 to move to coordinate (1000, 1000). The units of the coordinate system may be a function xcexc of the resolution of the inspection system. Merely by way of example, xcexc may be 40 nm in one type of inspection system, so that coordinate 1000 along the Y axis is actually at a location that is 40,000 nm away from the origin along that axis. As a result, if there is an error, an instruction to move to (1000,1000) actually may result in movement to coordinate (1000, 1000+xcex94y), where xcex94y is the amount of error. In the example just given, xcex94y may have a relation to xcexc. To correct for this kind of error, it would be desirable to be able to provide an amount of compensation for xcex94y, particularly as a function of xcexc.
This type of error, which is referred to as a xe2x80x9cstaticxe2x80x9d error, may not occur in certain types of stages, such as an interferometer controlled stage. Such an error may be an inherent function of the resolution of the stage control 25.
A second type of error, resulting from scan lines starting at different positions along the X axis, is referred to as xe2x80x9cdynamicxe2x80x9d error, because the amount of error can vary from scan to scan. One version of this dynamic error phenomenon is known as xe2x80x9cjitterxe2x80x9d. To compensate for jitter, it is necessary to monitor the starting position of the light beam along the Y axis, and provide appropriate compensation at the start of each scan line. FIG. 3 shows an example of different degrees of jitter. To compensate for jitter, the starting position for each scan line should be normalized to a position that is an amount xcex94x0 away from the Y-axis. For successive scans at times t1, t2, and t3, the jitter adjustment would be dx1, dx2, and dx3, respectively.
In order to correct for the kinds of static and dynamic error just described, it would be desirable to provide an approach which facilitates starting both the initial movement of the stage, and each scan line, at the same location every time.
One way of compensating for the foregoing types of misalignments and positioning errors is known as registration. In one such registration method, a scanned image is compared to a reference image. During this comparison, the scanned and reference images are aligned, to determine the existence of a positioning error. If differences between the scanned and reference images are detected during the comparison, then compensating factors may be determined.
In the kind of registration system just described, the reference image typically is obtained from a predetermined location on a scanning surface. Other systems may generate a reference image by utilizing a previously scanned image, or even by averaging a number of previously scanned images.
One drawback of the registration method is that registration accuracy relies heavily on the quality of the scanned images. As a result, if either the scanned image or the reference image is inaccurate, the errors detected as a result of the comparison will be incorrect. It can be appreciated readily that the problem will be even worse if both the scanned image and the reference image are inaccurate.
It would be desirable to provide a system that compensates for misalignment without the need for image comparison techniques such as the just-described registration method. It also would be desirable to provide accurate scanner misalignment correction without having to rely on the accuracy of sample or reference images.
In accordance with this and other aspects of the present invention, a method is provided that compensates for one or more sources of a scanner""s line alignment error. A typical surface inspection system may have a scanner with a scanning axis and a cross-scanning axis. In accordance with one aspect of the invention, when scanning a surface of an inspection object, a scanning axis signal may be output at predetermined distances along the scanning axis. The scanning axis signal may be used to determine the scanner""s speed.
Also in accordance with one aspect of the invention, the inspection system may be configured to output a jitter signal whenever the scanner deviates from the scanning axis. The jitter signal, which in one embodiment may take the form of a count signal, may be used to calculate the distance that the scanner deviates from the scanning axis.
Information such as the scanner""s speed, scan line resolution, and a scanning axis static position error may be used to generate a scan line that compensates for a scanning axis error. The generated scan line also may be shifted to compensate for a cross-scanning axis error based on the scanner""s jitter signal and a cross-scanning static position error.
In accordance with another aspect of the present invention, there may be a calibration session, during which a predefined grid on an inspection object may be scanned to determine a scanning axis static position error and a cross-scan line static position error for each point on the grid. Each of these errors may be loaded into respective static position error correction tables.
In accordance with another aspect of the present invention, an elapsed time between two consecutive outputted scanning axis signals may be measured to calculate the scanner""s speed.
In accordance with yet another aspect of the present invention, a cross-scanning axis error may be calculated by combining scanner deviation with cross-scanning static position error.
Another aspect of the present invention enables the system to determine the scanner""s resolution, which may be based on a variety of factors such as the scanner""s speed, or the inspection object""s surface structure or material composition.
Still another aspect of the present invention provides a system that may scan a variety of different types of inspection objects, such as semiconductor wafers, photomasks, reticles, flat panel displays, and biological materials.
In accordance with still yet another aspect of the present invention, a scanning process may continue until the scanning pass is completed.
These and other aspects, features and advantages of the present invention will become more apparent upon consideration of the following description of embodiments of the present invention taken in conjunction with the accompanying drawings.